Calling it the "however-you-want-it frappuccino," Starbucks will allow customers to order the drink with soy milk instead of dairy, decaffeinated instead of caffeinated coffee, and far fewer calories. The calorie count for a grande caramel light frappuccino, for instance, could drop 25% to 120 calories if ordered with nonfat milk, light syrup and no whipped cream.

A rollout years in the making, the customized frappuccino is set to make its debut in early May in the U.S. and Canada. Starbucks is hoping its sales will help offset the seasonal slowing of hot-coffee sales during summer. Any boost in frappuccino orders also would strengthen a weak portion of the Starbucks day—the afternoon, when iced drinks are most popular, the company said.

The launch has the potential to expand the coffee chain's customer base, said
Annie Young-Scrivner,
global chief marketing officer for Starbucks. The 15-year-old frappuccino has a strong history of luring newcomers to the Starbucks brand—particularly 18- to 24-year-olds, an age group for which the custom-made frappuccino is ideal, Ms. Young-Scrivner said.

Millennials, as these consumers are know, "are the iPod age group…accustomed to selecting exactly what they want," she said. Now they can choose an extra shot of espresso, no whipped cream or a dab more caramel, for instance.

There are risks. Any launch can flop, of course. And in the past, the frappuccino has wrought havoc in many locations by selling too well. Because it's mixed in a blender, topped with whipped cream and drizzled with toppings, the frappuccino takes longer than most Starbucks drinks to prepare. During the summer of 2006, when excessive heat drove up demand for frappuccinos, long lines scared customers away, and Starbucks reported slower-than-expected sales growth.

To prepare for custom-made frappuccinos, Starbucks is teaching its baristas a more-efficient production process and equipping them with more-efficient (and quieter) blenders.

During a recent test run of the product in Dallas, baristas had no trouble managing enthusiastic consumer response, Ms. Young-Scrivner said. The new approach "should not increase" the wait for drinks, she said.

Launched as an experiment in a California Starbucks, the frappuccino went national about 15 years ago and soon became a popular alternative to hot drinks. Annual U.S. frappuccino sales for Starbucks eventually hit $1 billion, helped in part by grocery sales of bottled frappuccino.

Over the years, as customers personalized every other drink on the Starbucks menu, the frappuccino remained resistant to modification. Although it came in six flavors, such as double chocolaty chip and coffee light, customers couldn't tweak the ingredients. That limitation helps explain why sales of the frappuccino have been softening the past two years, Ms. Young-Scrivner said.

Starbucks will launch the custom-made frappuccino primarily with in-store marketing. Baristas will encourage frappuccino customers to consider personalizing their orders.

The debut will follow by about seven months Starbucks' introduction of Via, an instant-coffee packet. During the fiscal first quarter ended Dec. 27, Starbucks reported growth in same-store sales for the first time since the end of 2008, a turnaround it attributed to Via's greater-than-expected success.

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